The present invention relates to a method for management of storage or “memory” regions in external memory device systems of computers and, in more particular, to a management method as to snap shots.
In memory devices such as fixed or “hard” disk drives (HDDs), it is required that data as recorded on a memory device be periodically copied for storage (backup) to another memory device such as a magnetic tape storage device for future use in the case of disappearance of data due to operation failure or obstruction of the memory device, software program defects, operation errors and the like. In such case, the data of interest for use as an object to be backed up (original data) will be updated during the backup; if mismatch is found between the backed up data and the data prior to execution of backup, then it will no longer be possible to reproduce any data at a time point that the backup gets started. Due to this, it is required to guarantee the matchability of data during copy operations.
To guarantee the matchability of data to be backed up, programs other than a backup program may be interrupted or halted while the backup program is being operating in a computer system. However, with a system under requirement for higher availability functions, it is impossible to interrupt the program for a long time. Due to this, there is required a systematic mechanism for forming in more than one backup-use memory device a layout location (to be referred to as “memory image” hereinafter) of data and the content of data as stored in a memory device at a backup startup time point while at the same time permitting any program to update data during backup operations. Here, let the memory image of data at a given time point be called a snap shot. In addition, let the mechanism for providing the state capable of updating data while forming a snap shot at any designated time point be called a snap shot management method. Additionally let the activity of forming a snap shot by the snap shot management method be called snap shot acquisition. Additionally let withdrawal of the state that has formed a snap shot be called snap shot deletion.
Prior known snap shot management methods include a method for duplexing or “mirroring” data.
With this method, in an ordinary or normal state in which any snap shots are not acquired yet, a program on a computer duplexes and stores all the data items in two separate memory regions. Upon acquisition of a snap shot, the duplexing or “mirroring” procedure is interrupted causing one region to be provided as the original data while letting the other region be provided as a snap shot.
In cases where update of the original data occurs during interruption of the mirroring, the position of any data thus updated is to be recorded. In a snap shot deletion event, data mirroring gets started again. In addition, based on a record of the position of such updated data, the updated data will be copied from the original data memory region to the other memory region. This copy operation will be called the mirror resynchronization. A method of mirroring data by use of a program on a computer is disclosed in U.S. Pat. No. 5,051,887 to Blaine H. Berger et al.
The above-noted two regions are generally constituted from a disk array in most cases. The disk array is arranged so that a plurality of disk devices are laid out into an array-like form. A memory device system including such disk array will be called the disk array apparatus hereinafter. The disk array apparatus is a memory apparatus which processes read and write requests from an upper-level or “supervisory” computer equipment (host) through parallel operations of disk devices at high speeds while at the same time improving the reliability by adding redundant data to the data being handled. The disk array can also be called the redundant array of inexpensive disks (RAID). Disk arrays are typically categorized into five levels depending upon the type and arrangement of the redundant data.
To ensure that the rate of access to a memory region with the original data being stored therein does not decrease during the use of a snap shot, the memory region of original data and the memory region of snap shot are formed of physically different disk-arrays respectively. In addition, let the arrangement of disk devices in a disk array be represented by “nD+1P,” where D is a disk device with data stored therein, and P is a disk device with redundant data stored therein. The formula nD+1P indicates the arrangement of n data recording disk devices and a single disk device with redundant data stored therein. Considering based on the arrangement of such disk devices, a disk array in which original data memory regions are laid out and a disk array in which memory regions used for snap shots are disposed are designed to have the same number of disk devices, that is, a configuration having the same n, wherein those being the same in storage capacity of individual disk devices are employed.
From several view points of performance and reliability plus costs or the like, different features are required for the memory region in which the original data is to be stored therein and for the memory region used for snap shots. One example is that the reliability is thought to be important in the memory region with the original data being stored therein whereas lower bit costs are required for the memory region used for snap shots. Additionally in some cases, higher performance is required for the memory region being used for snap shots in order to speed up the intended backup operations.
To be brief, it should be strictly required that each disk array be made up of specific disk devices different in performance, record density, storage capacity, reliability and the like. In this case, since the features required can be realized depending on the scheme for arranging the disk array (nD+1P), it is required that respective ones of the disk arrays with the original data memory region and the memory region used for snap shots being disposed therein be configured from nD+1P configurations having mutually different n values. However, in the prior art, it has been impossible or at least greatly difficult to realize any successful combination of disk arrays in which mutually flexible arrangements are employed.